“It’s a terrific performance, it’s the performance of his career,” Mr. Affleck said. He was speaking of Mr. Phoenix’s two-year portrayal of himself — on screen and off — as a bearded, drug-addled aspiring rap star, who, as Mr. Affleck tells it, put his professional life on the line to star in a bit of “gonzo filmmaking” modeled on the reality-bending journalism of Hunter S. Thompson.

“I’m Still Here” was released last week by Magnolia Pictures to scathing reviews by a number of critics, including Roger Ebert, who wrote that the film was “a sad and painful documentary that serves little useful purpose other than to pound another nail into the coffin.”

“The reviews were so angry,” said Mr. Affleck, who attributed much of the hostility to his own long silence about a film that left more than a few viewers wondering what was real — The drugs? The hookers? The childhood home-movie sequences in the beginning? — and what was not.

Virtually none of it was real. Not even the opening shots, supposedly of Mr. Phoenix and his siblings swimming in a water hole in Panama. That, Mr. Affleck said, was actually shot in Hawaii with actors, then run back and forth on top of an old videocassette recording of “Paris, Texas” to degrade the images.

“I never intended to trick anybody,” said Mr. Affleck, an intense 35-year-old who spoke over a meat-free, cheese-free vegetable sandwich on Thursday. “The idea of a quote, hoax, unquote, never entered my mind.”

Still, he acknowledged that Mr. Letterman was not in on the joke when Mr. Phoenix, on Feb. 11, 2009, seemed to implode his own career by showing up in character as a mumbling, aimless star gone wrong.

That was just three years after he had received an Oscar nomination for his spot-on performance as Johnny Cash in “Walk the Line,” and memories of the film were fresh enough to induce shock in the millions who watched him on the show and in later Internet replays.

Asked whether Mr. Phoenix would be in character for his return to Mr. Letterman’s program on Wednesday, Mr. Affleck said, “No, no, no.” And Mr. Letterman has not talked with Mr. Phoenix about the coming appearance, he added. Most mockumentaries, in the way of “This Is Spinal Tap,” wear their foolishness on their sleeves, leaving no doubt about their character as fiction. But Mr. Affleck, who is married to Mr. Phoenix’s sister and has been his friend for almost 20 years, said he wanted audiences to experience the film’s narrative, about the disintegration of celebrity, without the clutter of preconceived notions.

So he said little in interviews. “We wanted to create a space,” he said. “You believe what’s happening is real.”

As the film progresses, Mr. Affleck explained, subtle cues were supposed to provide hints of his real intention. Camera techniques, extremely raw at the beginning, become more sophisticated as the film goes on, for instance.

“There were multiple takes, these are performances,” Mr. Affleck said of unsettling sequences in which Mr. Phoenix appears to snort drugs, consort with hookers, and hunt to the ground an assistant who has betrayed him to the press — again, mostly actors.

But the movie never quite showed its hand. “There was no wink,” Mr. Affleck said.

One of the trickier elements was to win the cooperation of Mr. Phoenix’s agent, Patrick Whitesell, of William Morris Endeavor Entertainment. On telling Mr. Whitesell that he planned to make everybody believe that a prized client “has lost his mind and make him as unattractive as possible, you would think he would have me killed immediately,” Mr. Affleck said.

Mr. Phoenix’s unconventional background may have helped convince some that the film was true. Now 35, he was one of five children in a free-spirited family that bounced from life in a religious cult through a time when the siblings worked as street performers. Mr. Phoenix’s brother River, also an actor, died of a drug overdose in 1993. His sister Summer eventually married Mr. Affleck.

In the film Mr. Phoenix is often called “J. P.,” both an attempt at a rap stage name and the inevitable shorthand of a star’s inner world. At one point in the film Mr. Phoenix howls at his crew in exasperation: “J. P. is all of us.”

As Mr. Affleck now makes clear, he is actually none of us — which is something of a relief.

But Mr. Phoenix may now have his work cut out for him when it comes to repairing an image that was marred by what Mr. Affleck portrays as his best performance. The Los Angeles Times reported this week that Mr. Phoenix, who makes much of abandoning his screen career in the film, is fielding offers for new roles.

At least one element in the film was genuine, Mr. Affleck said. That was a snippet of a home movie that showed Mr. Phoenix and his very young siblings performing, Jackson Five style, on the streets of Los Angeles.

The rest, Mr. Affleck said, clearly requires a bit more understanding than he has allowed the viewers to date. “It is a hard movie to watch,” he said.

A version of this article appears in print on September 17, 2010, on page C1 of the New York edition with the headline: Documentary? Better Call It Performance Art. Order Reprints|Today's Paper|Subscribe